You may have this creature enter as a copy of any creature on the battlefield, except it enters with X additional +1/+1 counters on it.
Legal in
Pioneer, Modern, Legacy, Vintage, Commander, Brawl, Historic
Rulings (9)
You can choose not to copy anything. In that case, Altered Ego enters as a 0/0 creature and is probably put into the graveyard immediately. It won't have +1/+1 counters placed on it by its ability.
2025-01-24
Any "enters" abilities of the copied creature will trigger when Altered Ego enters the battlefield. Any "as [this creature] enters" or "[this creature] enters with" abilities of the chosen creature will also work.
2025-01-24
If Altered Ego somehow enters at the same time as another creature, Altered Ego can't become a copy of that creature. You may choose only a creature that's already on the battlefield.
2025-01-24
If the chosen creature is copying something else (for example, if the chosen creature is another Altered Ego), then your Altered Ego enters as whatever the chosen creature copied.
2025-01-24